Sliding of railway wheels travelling along a railway track occurs when the wheels on the corresponding rail vehicle are not rotating at the natural rotational speed equivalent to the vehicle's linear speed. When the wheels are braked either intentionally, or by a mechanical fault, beyond their limits of adhesion between the rails and the wheels, the wheels are said to be sliding.
Sliding wheels lead to damaged rails and wheels. Wheels that slide continuously can further lead to derailments of the associate vehicle or even the entire train. Further, wheels with worn tread do not track properly on the rails and lead to excessive rail and wheel wear.
A major challenge in the design of equipment to detect slipping or sliding wheels is the need for relatively inexpensive equipment which will operate dependably in the intensely hostile environment close to rail wheels.
Current systems are available which are based on vision equipment which takes repeated images of wheels and axles, and then by processing the image digitally measures forward velocity of the vehicle and the rotational velocity of the wheels. Assuming that the diameter of the wheels is known, the system can match the rotational velocity of the wheels to the forward speed of the vehicle thereby detecting slipping or sliding as a mismatch in these quantities.
This type of system is vulnerable to dust and rain, and also requires prior information about the rail wheel diameter. It is also difficult to apply to a number of vehicle designs where the wheel and axle rotation is difficult to detect visually.